The present invention relates generally to semiconductor thermal processing systems, and more specifically to a device and method for rapidly thermally processing a substrate.
Thermal processing of silicon substrates or wafers is important for manufacturing modern microelectronics devices. Such processes, including silicide formation, implant anneals, oxidation, diffusion drive-in and chemical vapor deposition (CVD), may be performed using conventional low temperature thermal processing techniques. In contrast, some dopant activation processes are performed at substantially higher temperatures for relatively short durations of time. Furthermore, many microelectronics circuits require feature sizes smaller than one micron and junction depths less than a few hundred angstroms. In order to limit both the lateral and downward diffusion of dopants, as well as to provide a greater degree of control during processing, it is desirable to minimize the duration of high temperature processing.
One approach for minimizing processing time utilizes a heat treatment apparatus such as a rapid thermal processing (RTP) system. Rapid thermal processing of semiconductor wafers provides a technique for fabrication of verylarge-scale-integrated (VLSI) and ultra-large-scale-integrated (ULSI) electronic devices. There are several challenges, however, to meeting the thermal requirements of rapid thermal processing. For example, fast rates of change of wafer temperature are typically desired, as well as temperature uniformity across the wafer during the temperature changes.
One significant performance limitation of conventional RTP systems (e.g., in terms of xe2x80x9cthermal budgetxe2x80x9d, or the time the wafer spends above about 950C) is rapidly switching between heating the wafer and cooling the wafer. Current RTP systems typically have a heat source that comprises either heat lamps or resistive elements for heating the wafer. Heat lamps have the benefit that the lamps can be quickly switched on and off, however, the thermal profile across the surface of the wafer can vary significantly. Resistive elements, on the other hand, can provide more uniformity in the thermal profile across the surface of the wafer, however, the resistive elements cannot typically be switched on and off rapidly. Thus, resistive element type systems may require a mechanical shield to be placed between the wafer and the heater after heating is complete, and/or necessitate a movement of the wafer to or from the proximity of the heater.
In addition, rapid thermal annealing (RTA, also called xe2x80x9cspike annealingxe2x80x9d) requires the switching between heating and cooling to occur very quickly once the spike temperature is reached. Spike annealing of a semiconductor wafer typically comprises inserting the wafer into a thermal processing system, rapidly heating the wafer to a high temperature, and then quickly cooling the wafer back down to room temperature.
According to current technological requirements, an ideal heat source should be capable of rapidly increasing a temperature of the wafer to a maximum temperature of 1050C, and then to cease adding energy to the system, whereby the wafer could be rapidly cooled. Accordingly, temperature ramp rates in excess of 250 C/s are desirable, and present RTP industry forecasts call for rates as high as 500 C/s in the near future. Typical lamp-based RTP systems rely primarily on radiative heat transfer. Typical lamp-based systems generally suffer from both temperature profile uniformity issues due to their numerous lamps across a surface of the wafer, as well as wafer pattern effects due to the radiative nature of the heat transfer. The patterning effects are typically caused by varying emissivities across the surface of the wafer, wherein patterned polysilicon, nitride, and oxide, for example, absorb the radiative heat at different rates, thus producing micro-scale temperature gradients on the device level.
FIG. 1 illustrates a typical resistive heating enclosure 10, wherein some of the issues related to the patterning effects seen in a typical lamp-based system are resolved by heating the substrate 15 using a heated block 20. The enclosure 10 typically comprises a resistive heater 25 configured to heat the heated block via conduction, wherein the heated block attains a generally uniform thermal profile at a surface 30 associated with the substrate 15. The substrate 15 resides on a lift mechanism 35 (typically comprising a plurality of pins 40), wherein the lift mechanism is operable to translate the substrate between a heating position 45 proximate to the heated block 20 and a loading position 50. The substrate 15 is generally radiatively heated by radiative heat transfer from the heated block 20, as well as by conductive or convective heat transfer through a gas (not shown) residing within the enclosure 10.
The prior art heating enclosure 10, however, suffers several difficulties when attempting to attain the fast temperature ramp rates required in RTA processes. For example, the heated block 20 continues to transfer radiative thermal energy to the substrate 15, even when the substrate is in the loading position 50. Typically, this necessitates the loading position 50 to be a great distance away from the heated block 20 in order to aver the effects of the radiation. Such an arrangement is not optimal, however, since translation time between the heating position 45 and the loading position 50 must be accounted for in terms of thermal budget. Alternatively, a shield may be placed between the substrate 15 and the heated block 20 when such radiation is not desired. This solution, again, is not optimal, since it incorporates adding another moving part within the enclosure 10 that may create contamination issues. Furthermore, wafer insertion into the thermal processing system can pose other problems, including non-uniform heating of the wafer due to the time needed for placement of the wafer. Non-uniform heating of the wafer can lead to severe mechanical stress in the substrate, thus negatively impacting yield or even rendering the wafer useless.
Therefore, a need exists in the art for a heat source that can be readily switched between a low temperature for wafer insertion and removal and a high temperature for rapid heating of the wafer, while also providing temperature uniformity across the wafer.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention is directed to a thermal device for processing a substrate, as well as a method for thermally processing a substrate. The thermal device is operable to provide a rapid and uniform temperature change to efficiently heat a substrate, thereby improving process control. According to one aspect of the present invention, a container is disclosed comprising a heat source and a plurality of thermal shields. The plurality of thermal shields, for example, are spaced from one another by a predetermined distance, defining one or more gaps therebetween. The predetermined distance is furthermore associated with a mean free path of a gas residing within the one or more gaps.
In accordance with one aspect of the present invention, the predetermined distance that separates the plurality of thermal shields, for example, is sized such that thermal conduction between the plurality of thermal shields via the gas is generally permitted in the free molecular regime. In one example, the plurality of thermal shields comprise one or more perforations therethrough, wherein the one or more perforations are operable to allow a flow of the gas freely therethrough. The perforations, for example, are substantially larger than the predetermined distance, wherein the perforations generally permit the gas to flow in a viscous regime, thereby facilitating an expedient flow of gas within the one or more gaps. The gas, for example, is introduced into the one or more gaps from a gas source, or from a source such as from within a process chamber atmosphere.
According to another exemplary aspect of the present invention, the plurality of thermal shields are generally corrugated, wherein the predetermined distance is selectively variable. In one example, the predetermined distance is selectively variable based on a pressure differential between an interior portion of the container and an external environment. The pressure differential is operable to generally compress or expand the plurality of thermal shields with respect to one another, thereby respectively decreasing or increasing the predetermined distance associated with the one or more gaps. Increasing the predetermined distance generally facilitates thermal conduction through the gas in the viscous regime, while decreasing the predetermined distance generally limits thermal conduction through the gas in the viscous regime.
According to another exemplary aspect of the present invention, a method for thermally processing a substrate in a thermal device is disclosed, wherein the thermal processing system comprises a container further comprising a heat source and a plurality of thermal shields. The method comprises initiating a gas at a first pressure within the one or more gaps defined by a predetermined distance between the plurality of thermal shields, wherein a gas residing within the one or more gaps is generally non-conductive. A substrate is placed in the thermal device and a heating position is initiated, wherein the substrate is in close proximity to a surface of the container. The gas is introduced into the one or more gaps at a second pressure, wherein thermal conduction is generally permitted between the plurality of thermal shields, and wherein the substrate is exposed to thermal radiation and thermal conduction in the free molecular regime from the heat source. Alternatively, when the gas is introduced at the second pressure, the predetermined distance increases, wherein the substrate is exposed to thermal radiation and conduction in the viscous regime. The first pressure is again initiated, wherein the thermal conduction is essentially halted, and the substrate is removed from the thermal device.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.